It has beta shown that the RNA plant virus, brome mosaic virus (BMV), can replicate in the yeast S cerevuae. However, different physical properties inclicated that there were several distinct species of virus-like particles (VLPs). Cryo-EM studies indicated that some of the particles were "normal" BMV-Iike particles but others were smaller particles of an unusual architecture. Mass measurements using the STEM should help in modeling the structure. Preliminary mass measurements are quite good, and there do appear to be at least two classes of VLP, although the ma of the control BMV was lower than expected. There are also some unusual fiber-appearing particles.